EP1019153A1

EP1019153A1 - Agent for eliminating heavy metals comprising a phosphate compound

Info

Publication number: EP1019153A1
Application number: EP98944012A
Authority: EP
Inventors: Yves Mottot; J.-Claude Lotissement Le Grand Caillouet KIEFER
Original assignee: Rhodia Chimie SAS
Current assignee: Rhodia Chimie SAS
Priority date: 1997-09-30
Filing date: 1998-09-17
Publication date: 2000-07-19
Anticipated expiration: 2018-09-17
Also published as: US6444183B1; FR2768933A1; KR100400503B1; JP2001518376A; KR20010030815A; FR2768933B1; AU9169898A; DE69804646D1; DE69804646T2; ATE215392T1; EP1019153B1; ES2175770T3; JP3579654B2; WO1999016509A1

Abstract

The invention concerns an agent for eliminating heavy metals contained in an aqueous effluent comprising: an alkaline metal silicate, an alkaline metal carbonate; a phosphate compound, for example a sodium orthophosphate: and optionally a carrier, in particular a clay. Said agent may further contain a sulphur compound. Said agent can constitute an agent stabilizing said metals, and can be used for eliminating or stabilizing heavy metals, in particular cadmium, nickel and lead, contained in industrial aqueous effluents.

Description

HEAVY METAL REMOVAL AGENT COMPRISING A PHOSPHATE COMPOUND

The present invention relates to an agent for removing heavy metals contained in a medium, in particular an aqueous effluent.

Legislation in the field of heavy metal releases to the natural environment is constantly evolving. Industrial effluents such as, for example, those from factories in the chemical, metallurgical, electronic, mechanical industries and the water used to wash (or purify) fumes from incineration plants for household or industrial waste, particularly 'industrial effluents of the used sulfuric acid type are media likely to contain heavy metals. Similarly, some soils are polluted by the presence of such metals.

Thus, in the field of water for washing smoke from household waste incineration plants, the best-known process for removing heavy metals consists of basic precipitation carried out with lime; the subsequent decantation / separation step is generally improved by the incorporation of a flocculating agent. However, this process has a certain number of drawbacks. On the one hand, this type of effluent to be treated often has a very high content of mineral salts, which can hinder the complete precipitation of heavy metals. On the other hand, a very large volume of mud is generated by lime precipitation; this sludge, after filtration and compaction in the form of a cake, must currently be placed in suitable landfills.

In addition, the mud composition obtained can be difficult to stabilize (or immobilize) by current techniques. However, future regulations relating to the storage of ultimate special waste make it necessary to stabilize (or immobilize) the cake, before admission to the storage site, in order to very significantly reduce the leaching of this type of waste.

The object of the present invention is in particular to provide a means allowing very efficient elimination (or slaughter) of heavy metals, in particular cadmium, nickel and lead, as well as, in certain cases, mercury, and not having the disadvantages mentioned above.

To this end, the present invention provides a new agent for eliminating (or slaughtering) heavy metals present in a medium, said agent being a mixed product comprising at least one alkali metal silicate, at least one alkali metal carbonate, at least one compound of the phosphate type, and, optionally, at least one support, in particular a clay. It also relates to the use of said agent for the removal of heavy metals, in particular cadmium, nickel and lead, as well as, in certain cases, mercury, contained in a medium, in particular an aqueous effluent.

It also relates to an agent for stabilizing (or immobilizing) heavy metals, comprising said agent for removing heavy metals.

The Applicant has surprisingly found that the use of a heavy metal removal agent as defined above allows very effective removal of heavy metals from the medium containing them, in particular cadmium, nickel and lead, and, when it comprises a sulfur compound, mercury, and, advantageously, an improvement in the settling and separation of the sludge obtained / liquid supernatant, a certain improvement in the ability to stabilize this sludge, it is ie a certain improvement in the ability of the latter to resist leaching and a reduction in the residual calcium content of the precipitate compared to what is obtained with the lime process described above. Thus, one of the objects of the invention is an agent for removing (or capturing) heavy metals contained in a medium, characterized in that said agent comprises:

at least one compound consisting of an alkali metal silicate, hereinafter called compound (A),

at least one compound consisting of an alkali metal carbonate, hereinafter called compound (B),

at least one compound of the phosphate type, hereinafter called compound (C), and

- possibly, at least one support.

By heavy metals is meant in particular the metals of valence greater than or equal to 2, preferably equal to 2, and in particular those chosen from antimony, arsenic, bismuth, cadmium, chromium, cobalt, copper, tin, manganese, mercury, molybdenum, nickel, gold, lead, thallium, tungsten, zinc, iron, metals of the actinide family.

The heavy metals particularly targeted by the present invention are chromium, copper, iron, zinc, and, to a greater degree, cadmium, nickel, lead. Certain embodiments of the agent according to the invention are advantageously used when the medium to be treated contains, as heavy metals, at least mercury.

The heavy metals to be removed are usually found in the form of ions, in particular in the form of their respective cations (for example Cr ^{3 +} , Cu ^{2 +} , Ni ^{2 +} , Fe ^{2 +} , Fe ^{3 +} , Cd ^{2 +} , Hg ^{2 +} , Pb ^{2 +} , Zn ^{2 +} ). It should be noted that the agent according to the invention can also prove to be effective for the removal of metals such as aluminum and of so-called radioactive metals such as uranium or thorium.

The medium to be treated is preferably liquid. This medium can thus be constituted by an aqueous effluent, in particular an industrial aqueous effluent (that is to say an aqueous effluent originating from an industrial process).

This medium may for example be an aqueous effluent formed by the water from washing (or purifying) the waste incineration fumes, in particular household waste, industrial waste, hospital waste, by material washing water. solid, like earth, containing heavy metals, by aqueous effluents from surface treatment; it can be an aqueous effluent from a factory in the chemical, metallurgical, electronic, mechanical industry.

Compound (A) is an alkali metal silicate, in particular sodium or potassium.

Compound (A) is advantageously a sodium silicate. Said sodium silicate then generally has a Siθ2 / Na2θ molar ratio of between 0.5 and 3.8, for example equal to around 2.

Compound (B) is an alkali metal carbonate, in particular sodium or potassium. Compound (B) is advantageously a sodium carbonate.

The weight ratio between compound (B) and compound (A) can vary within relatively wide ranges of values. However, this compound (B) / compound (A) weight ratio is generally between 0.5 and 3.5, preferably between 1, 1 and 2.5, in particular between 1.5 and 2.0. When it contains at least one support, the mixed product for removing heavy metals from a medium according to the invention, which can be used to purify said medium, can be considered as a composite product formed:

- at least one active principle (or precipitation agent) consisting of:

. at least one compound (A) consisting of an alkali metal silicate,. at least one compound (B) consisting of an alkali metal carbonate,

. at least one compound (C) of the phosphate type; and

- at least one support (or substrate).

The support possibly contained in the agent according to the invention is, preferably, a clay. The clay then contained in the agent according to the invention can be of natural or synthetic origin.

The clay thus used in the agent according to the invention advantageously has a high content by weight of Al2O3; this content is for example between 20 and 40%. It is possible to use, according to the invention, a clay of lamellar or phyllosilicate structure.

It is thus possible to use a clay chosen from the group comprising kaolinites, serpentines. The clay can also be chosen from the group comprising montmorillonites, bentonites (in particular alkaline), talc, mica.

Preferably, the clay chosen belongs to one of these two groups.

It is optionally possible to use a clay of chlorite type structure. The agent according to the invention generally contains, as clay, a montmorillonite or a bentonite.

The content of support, in particular clay, of the agent according to the invention is usually between 5 and 90% by weight, for example between 10 and 35% by weight, relative to the total weight of said agent. The presence of a phosphate compound (C) in the agent according to the present invention is essential.

This compound (C) is an inorganic compound, advantageously containing (or providing) at least one phosphate function.

It can be chosen from orthophosphates, pyrophosphates, tripolyphosphates, metaphosphates, hypophosphates, monophosphates, diphosphates of alkali metal, in particular sodium or potassium, or of alkaline earth metal, in particular magnesium.

In particular, it is possible to use a potassium diphosphate or a pyrophosphate.

The use of magnesium orthophosphate (Mg ₃ (PO ₄ ) ₂ , xH ₂ O) may prove to be advantageous, in particular because of its very low solubility, which may result in a residual phosphate content of the medium, after treatment with the agent according to the invention, also very low.

Preferably, the compound (C) is a sodium pyrophosphate (Na ₄ P ₂ O ₇ , xH ₂ O), a sodium tripolyphosphate (Na ₅ P ₃ O ₁₀ , xH ₂ O) or, even more preferably, sodium orthophosphate (Na ₃ PO ₄ , xH ₂ O).

The content of compound (C) of the agent according to the invention can be between 1 and 30% by weight; it is preferably between 5 and 20% by weight.

A particularly interesting example of agent according to the invention has the following composition: sodium silicate / sodium carbonate / sodium orthophosphate.

When the medium to be treated contains mercury, then, if it is desired to destroy it more effectively, the agent according to the invention advantageously further comprises at least one sulfur-containing compound (D).

This compound (D) can be a mineral sulfur compound. In general, the agent according to the invention then does not contain a support, in particular clay.

As an inorganic sulfur compound, an inorganic sulfide can be used, in particular a barium sulfide (BaS) or strontium (SrS), or, preferably, a mineral (poly) thiocarbonate, in particular an alkali metal (poly) thiocarbonate, for example potassium or sodium. It is thus possible to use any salt of thiocarbonic acid, such as potassium thiocarbonate (K2CS3).

However, the compound (D) is preferably an organic sulfur compound (or so-called organosulfur compound).

The agent according to the invention then preferably contains at least one support, in particular a clay.

As organic sulfur compound, one can use an organothiophosphate or an organodithiophosphate, in particular a dialkyl- or diaryl-dithiophosphate of alkali metal, for example of sodium.

The alkali metal dialkyl- or diaryl-dithiophosphates which can be used correspond in particular to the following formula:

in which X is an alkali metal, for example sodium, R is an aryl or, preferably, alkyl radical, for example, methyl, ethyl, n-propyl, isopropyl, 1-methyl propyl, isobutyl.

As organic sulfur compound, it is possible to use, preferably, a (poly) mercapto compound, in particular a mercapto, dimercapto or trimercapto compound. This organic sulfur compound can then be a triazine (for example a triazine-as or, preferably, a triazine-S), substituted by 1, 2 or 3 monovalent radicals -SH. More particularly, this organic sulfur compound is trimercapto triazine-S, which corresponds to the following formula:

The sulfur-containing compound (D) is preferably located, in the support, at the heart of the agent according to the invention.

This localization at the heart of the mixed product according to the invention allows in particular a delayed dissolution of the sulfur compound (D) during the use of said mixed product in a liquid medium; the Applicant has found that the dissolution of the sulfur compound (D) occurs with a certain delay with respect to the dissolution of the constituents (A), (B) and (C).

This delayed dissolution has the particular advantage of reducing the necessary quantity of sulfur compound in the agent according to the invention. The content of sulfur compound (D) of the agent according to the invention can be between 0.01 and 5%, in particular between 0.1 and 4%, by weight; it is preferably between 0.4 and 3%, for example between 0.5 and 2%, by weight.

The agents according to the invention, which contain the compounds (A), (B), (C), and, optionally (but not preferentially) a support, in particular a clay, are prepared by any suitable process.

Compound (B) in solid form and compound (C) in solid form (and optional support) are added, generally with stirring, to a granulator, preferably a mixer granulator, and the mixture is sprayed (generally with stirring ) in said granulator an aqueous solution of compound (A), in particular at a spraying pressure of between 5 and 15 bars; granulation can be carried out using a plate granulator or a Lodige type granulator. The aqueous solution of compound (A), the pH of which is usually between 10 and 14, can be heated beforehand in particular to a temperature between 60 and 90 ° C, for example between 70 and 85 ° C. If the compound (A) is a sodium silicate, an aqueous sodium silicate solution is generally used having a molar ratio Siθ2 Na2θ of between 0.5 and 3.8, for example equal to about 2 and a concentration of silicate , expressed as Siθ2, between 0.1 and 10 mol / l, for example between 0.2 and 8 mol / l.

This mixing step (spraying in particular) is preferably followed by drying, generally in a rotary dryer, usually between 30 and 40 ° C, then a new mixture with an aqueous solution of the compound (A), mixture advantageously consisting of a spraying, under conditions as described above, of said solution onto the product resulting from this last drying, previously reintroduced into a granulator (for example that used before), preferably a mixer granulator.

An intermediate sieving step after the first drying and / or a final sieving step can be implemented according to the desired particle size.

To prepare an agent according to the invention further comprising a sulfur compound

(D), any suitable process is implemented which preferably makes it possible to introduce the sulfur-containing compound at the heart of said agent. A particularly preferred method when it is then desired to prepare an agent containing a support, such as a clay, will be described below.

The sulfur compound (D) is first incorporated into the heart of the support. For this, an aqueous solution of the sulfur-containing compound (D) is added, generally with stirring, to a support, in particular a clay, previously introduced into a granulator, preferably a mixer granulator. Granulation can be carried out using a plate granulator or a Lodige type granulator. The granulation is then completed by drying in a fluid bed, preferably at a temperature between 40 and 100 ° C., in general at a value kept constant for a certain time after a rise in temperature. This temperature can be between 40 and 70 ° C., for example between 45 and 55 ° C. It can also be between 70 and 100 ° C., in particular between 85 and 95 ° C. The delayed dissolution of the sulfur compound (D) when using the agent according to the invention in a liquid medium is also due, in general, to this heat treatment. The temperature of the heat treatment can constitute a means of controlling the kinetics of dissolution of the sulfur compound (D).

The dried product obtained, in the form of granules, can then be ground (in order to carry out in particular a stripping of said granules) and, optionally, sieved (with, for example, recovery only of the fraction of granules of size less than 400 μm, or even at 200 μm).

Following this incorporation of the sulfur-containing compound (D) at the heart of the support, the preparation of the agent according to the invention can continue as described, by replacing, in the corresponding preceding description, support by support + compound (D).

The agent according to the invention is usually in the form of granules (co-granules) or powder, the grain size of which is preferably between 0.1 and 2.0 mm, in particular between 0.2 and 1, 6 mm.

It can nevertheless be in a liquid form, after having been suspended in water; no support then generally enters into the composition of the agent.

In general, the agent according to the invention has a weight content of free water (or humidity) of between 10 and 30%, preferably between 15 and 20%. This water content can be determined by measuring mass loss by calcination at 350 ° C for 6 hours.

The use of at least one agent according to the invention for removing heavy metals from a medium containing them, in particular a liquid effluent (or solution), in particular an aqueous effluent (or solution), can be implementation in the following manner. The agent according to the invention is introduced into the liquid effluent to be treated, with stirring. The final pH of the suspension containing said agent which has been added to it is preferably between 7 and 11 or adjusted to a value between 7 and 11 by prior addition of a base or an acid; said pH can be in particular (adjusted) around 9. The final pH depends on the amount of agent according to the invention introduced into the liquid effluent to be treated and the initial pH of said effluent. Agitation is continued, for example for 5 to 60 minutes. The suspension can then be left to settle (decant) at room temperature for a certain time, generally between 0.5 and 24 hours, in particular between 0.5 and 6 hours. The settling time can be reduced if rapid settling methods known to those skilled in the art are used. Then, the precipitate formed is separated, that is to say the agent according to the invention loaded with heavy metals, in particular cadmium, nickel, lead, by decantation, filtration and / or centrifugation of the suspension.

Generally, the medium to be treated, in particular when it consists of a liquid effluent (in particular an aqueous effluent (a solution)), contains 0.5 to 6000 mg / l, for example 1 to 1000 mg / l, in particular 2 to 300 mg / l of heavy metals.

The amount of agent according to the invention added to the medium to be treated is such that the molar ratio (Siθ2 + Cθ3 ^{2 "} ) / (cations present in the medium to be treated) is, in general, between 0.7 and 2, 5, for example between 1.0 and 2.2, in particular between 1.1 and 1.9, here the term cations present in the medium to be treated is understood to mean cations of heavy metals and cations Ca ²⁺ .

The use of agents according to the invention advantageously allows very effective removal of heavy metals, in particular cadmium, nickel and lead, or even mercury for certain embodiments of the invention, in particular in a fairly wide pH range, usually between 7 and 11.

In addition, it is found that, after separation, the precipitate formed, that is to say the agent according to the invention loaded with heavy metals, preferably has a satisfactory ability to stabilize (or immobilize). It exhibits very acceptable behavior with respect to leaching: it is in fact not very leachable, that is to say that it hardly or hardly releases heavy metal cations which it contains when it is placed in the presence of water; the quantities of heavy metal chemical species in leachate obtained from conventionally carried out leaching tests are relatively small.

Another object of the invention thus consists of a stabilizing (or immobilizing) agent for heavy metals contained in a medium, characterized in that it comprises at least one agent as described above.

If the presence of a support in the agent according to the invention allows, in general, localized precipitation on the periphery of said support, it also makes it possible, in particular in the case of clay, to favor, surprisingly, in the case where the agent according to the invention comprises a compound of the carbonate type, decantation, in particular to increase the decantation speed. Likewise, it can also make it possible to reduce the low contents of heavy metals in the leachate as mentioned above. It is particularly interesting when the agent is used especially in finishing, that is to say for the elimination of the last traces of heavy metals.

The following examples illustrate the invention without, however, limiting its scope.

EXAMPLE 1

1) 4.7 kg of Na ₂ CO ₃ and 0.8 kg of sodium orthophosphate (Na ₃ PO ₄ , 12H ₂ O) are introduced into a Lodige mixer granulator with a capacity of 20 liters; the mixture is homogenized at 160 revolutions / min (coulter speed), for 20 minutes.

Then sprayed on the mixture located in the mixer granulator operating at the same speed (160 rpm) 4.0 kg of a sodium silicate solution, previously heated to 75 ° C, molar ratio SiO ₂ / Na ₂ O equal to 2, silicate concentration, expressed as SiO ₂ , of 30.6% by weight, at a spraying pressure of 12 bars and at a flow rate of 13.5 l / h.

Granules are recovered which are then dried, for 45 minutes, in a rotary dryer, using hot air so that the temperature of the granules does not exceed 50 ° C.

The granules are then sieved (using a 1.6 mm sieve); the refusal is ground using a FORPLEX pin mill, fitted with a 3 mm grid. The sieved granules and the ground granules are reintroduced into the mixer granulator.

Finally, on these granules, being in the mixing granulator put back into service, 1.8 kg of a sodium silicate solution as defined above are sprayed (ie, in total of the two sprays, 2.6 kg of dry sodium silicate). Then, as indicated above, the granules obtained are dried.

The granules having a size between 0.2 and 1.6 m are then recovered by sieving; their average size is 0.7 mm.

The product obtained (P1) constituted by said granules comprises approximately 32% of sodium silicate, 58% of sodium carbonate and 10% of sodium orthophosphate.

EXAMPLE 2

An aqueous effluent of washing water from the incineration fumes of household waste is reconstituted as follows. Dissolve in 5 liters of aqueous sodium chloride solution containing

200 g NaCI the following quantities: Na ₂ SO ₄ 7.40 g CuCI ₂ 0.07 g

CaCI ₂ , 2H ₂ O 18.40 g CdCl _a 0.04 g

AICI ₃ , 6H ₂ O 2.23 g NiCI ₂ 0.10 g

FeCI ₃ , 6H ₂ O 1, 22 g PbCI, 0.07 g

ZnCI, 1.57 g

The pH of this effluent is adjusted to the value 2 by adding hydrochloric acid.

Said effluent has the following contents expressed in mg / l: Ca ^{2 +} : 1000 Cu ^{2 +}

Al ^{3 +} : 50 Cd +

Fe ^{2 +} + Fe ^{3 +} : 50 Ni ^{2 +}

Zn ^{2 +} : 150 Pb ^{2 +}

Two tests to remove the heavy metals contained in this effluent are carried out with:

the product P1 according to the invention, prepared in Example 1,

- A product P2 not in accordance with the invention, which differs from P1 by the absence of sodium orthophosphate (weight composition of P2: 35.5% sodium silicate and 64.5% sodium carbonate). These two products are used (separately) for a final neutralization pH of 9.

The quantities of product P1 and P2 to be used for 500 ml of said effluent are the following:

P1: 4.5 g for a final pH of 9, P2: 4.2 g for a final pH of 9.

For each test, the quantity of product P1 (or P2) indicated above is introduced instantaneously, with stirring, into 500 ml of said effluent and stirring is continued for 30 minutes.

The suspension obtained is then left to stand, at room temperature, for 4 hours.

Then it is centrifuged at 3000 rpm for 10 minutes. The following concentrations are measured (Table 1) in various species of the purified effluent (that is to say the supernatant) by any appropriate method (in particular ICP / MS (plasma emission spectroscopy with mass detection (apparatus used : ELAN 5000, PERKIN ELMER)) for Cd and Pb elements, and ICP / OES (plasma emission spectroscopy with optical detection (apparatus used: SOPRA DPS 1500)) for Ca, Al, Fe, Zn, Cu and Or).

TABLE 1

The results indicated in the table above show the very good effectiveness of the agent according to the invention; they illustrate in particular its better efficiency with respect to cadmium, nickel and lead compared to a product free of phosphate-type compound.

It is also worth noting the very significant reduction in the calcium level using the agent according to the invention. In the case of the treatment of an incinerator effluent, this may be of interest when said agent is used upstream of a stage of concentration or purification of sodium chloride by a process using resin or membrane .

Claims

1. Agent for removing heavy metals from a medium, characterized in that said agent comprises: - at least one compound (A) consisting of an alkali metal silicate,

- at least one compound (B) consisting of an alkali metal carbonate,

- at least one compound (C) of the phosphate type,

- possibly, at least one support.

2. Agent according to claim 1, characterized in that said compound (A) is a sodium or potassium silicate.

3. Agent according to one of claims 1 and 2, characterized in that said compound (B) is a sodium carbonate.

4. Agent according to one of claims 1 to 3, characterized in that said compound (A) is a sodium silicate and said compound (B) is a sodium carbonate.

5. Agent according to one of claims 1 to 4, characterized in that the weight ratio of compound (B) / compound (A) is between 0.5 and 3.5, preferably between 1.1 and 2, 5.

6. Agent according to one of claims 1 to 5, characterized in that said compound (C) is chosen from orthophosphates, pyrophosphates, tripolyphosphates, metaphosphates, hypophosphates, monophosphates and diphosphates of alkali metal or alkaline earth metal.

7. Agent according to one of claims 1 to 6, characterized in that said compound (C) is a sodium or magnesium orthophosphate, a sodium or potassium pyrophosphate or a sodium tripolyphosphate.

8. Agent according to one of claims 1 to 7, characterized in that said compound (C) is a sodium orthophosphate.

9. Agent according to one of claims 1 to 8, characterized in that it has a content of compound (C) of between 1 and 30%, preferably between 5 and 20%, by weight.

10. Agent according to one of claims 1 to 9, characterized in that it comprises at least one support, said support being a clay.

11. Agent according to one of claims 1 to 10, characterized in that it has a support content of between 5 and 90% by weight, in particular between 10 and 35% by weight.

12. Agent according to one of claims 1 to 11, characterized in that it also comprises at least one sulfur compound (D), preferably at a content of between 0.01 and 5%, in particular between 0, 1 and 4%, by weight.

13. agent for stabilizing heavy metals contained in a medium, characterized in that it comprises at least one agent according to one of claims 1 to 12.

14. Use of at least one agent according to one of claims 1 to 12 for the elimination of heavy metals contained in a medium.

15. Use according to claim 14, characterized in that said heavy metals are chosen from cadmium, chromium, copper, nickel, lead, zinc and iron.

16. Use according to one of claims 14 and 15, characterized in that said medium contains at least cadmium, nickel and lead.

17. Use according to one of claims 14 to 16, characterized in that said medium contains at least mercury.

18. Use according to one of claims 14 to 17, characterized in that said medium is an aqueous effluent, in particular an industrial aqueous effluent.

19. Use according to claim 18, characterized in that said medium is an aqueous effluent formed by the washing waters of the waste incineration fumes.